1. Field of the Invention
The present invention relates to a solid electrolyte and a method of producing the same, and particularly relates to a solid electrolyte having high ion conductivity and low reactivity with an electrode material and a method of producing the same.
2. Description of the Background Art
Conventionally, a lithium secondary battery using an organic electrolytic solution has been put into practical use. Such a lithium secondary battery using an organic electrolytic solution is characterized in that it has higher energy output per unit volume or per unit mass when compared with other batteries. Accordingly, the lithium secondary battery using an organic electrolytic solution has been developed and put into practical use to serve as a power source designed for mobile communication equipment, a notebook-sized personal computer, and an electric vehicle.
As to the conventional lithium secondary battery using an organic electrolytic solution, the organic electrolytic solution is basically a flammable substance. Accordingly, there arises a problem of temperature rise in the organic electrolytic solution, or a problem of incurring a risk of explosion of the lithium secondary battery when an impact is exerted on the organic electrolytic solution.
As to the lithium secondary battery using an organic electrolytic solution, there also arises a problem of incurring the risk of its explosion when a lithium-containing metal or the like is used for its negative electrode to improve energy density. This is because the lithium-containing metal may be precipitated on a surface of the negative electrode and grown in a dendrite form while the battery is repeatedly charged and discharged, resulting in that the lithium-containing metal grown in a dendrite form may cause a short circuit between the positive electrode and the negative electrode.
Accordingly, there has recently been considered the use of a solid electrolyte instead of the organic electrolytic solution used in the conventional lithium secondary battery, and there has been conducted a study of a solid electrolyte having high lithium ion conductivity and chemical stability.
When such a solid electrolyte is used for the lithium secondary battery, it is possible to overcome the above-described problems of the lithium secondary battery using an organic electrolytic solution, and additionally, obtain a stable operation even in a severe environment such as not lower than 200° C. or not higher than −20° C., which stable operation was difficult to obtain in the conventional lithium secondary battery using an organic electrolytic solution.
For example, Patent Document 1 (Japanese Patent Laying-Open No. 2002-184455) discloses a method of forming a solid electrolyte thin film having lithium and sulfur as essential constituents and containing at least one element selected from the group consisting of phosphorus, silicon, boron, germanium, and gallium. According to the method disclosed in Patent Document 1, there is performed a step of heating the solid electrolyte thin film to not lower than 40° C. and not higher than 200° C. to improve ion conductivity.
Furthermore, Patent Document 2 (Japanese Patent Laying-Open No. 2002-109955) discloses sulfide-based crystallized glass (solid electrolyte) obtained by calcining sulfide-based glass having Li2S and P2S5 as major constituents. This sulfide-based crystallized glass (solid electrolyte) is composed of a glass phase having Li2S and P2S5 as major constituents and a crystal phase containing a sulfide.
Patent Document 3 (Japanese Patent Laying-Open No. 2005-228570) discloses sulfide-based crystallized glass (solid electrolyte) having a particular crystal structure obtained by calcining at 150-360° C. sulfide-based glass having a composition of 68-74 mol % of Li2S and 26-32 mol % of P2S5.
In any of the above-described Patent Documents 2 and 3, calcination at a temperature not lower than a glass transition temperature is required for crystallization of the sulfide-based crystallized glass (solid electrolyte).
Furthermore, each of Non-Patent Document 1 (Solid State Ionics 170 (2004) pp. 173-180) and Non-Patent Document 2 (Electrochemical and Solid-State Letters 8 (11) A603-A606 (2005)) discloses a solid electrolyte made of Li2S and P2S5. Non-Patent Document 1 discloses in FIG. 2 on page 176, and Non-Patent Document 2 discloses in FIG. 3, X-ray diffraction patterns of the solid electrolytes, respectively, obtained by an X-ray diffraction method.